1. Field of the Invention
The present invention relates to a method for controlling a magnetic sound of an alternating current rotating machine.
2. Description of the Related Art
In recent years, electric cars, hybrid cars, fuel cell cars, etc. have reached the practical level or developmental level. In these cars, large output alternating current rotating machines are becoming the main units for generating the drive power, but such large output alternating current rotating machines suffer from the problem that they produce loud magnetic sound. As a method for reducing this magnetic sound, Japanese Unexamined Patent Publication (Kokai) No. 11-341864 proposes to generate current waveforms for canceling out the magnetic vibrating force based on information of fluctuation of the force so as to reduce the magnetic sound.
The principle of the technology for reducing magnetic sound by superposition of current of Japanese Unexamined Patent Publication No. 11-341864 described above can be easily understood, but it is not clear which waveforms of current to actually superimpose to reduce the magnetic sound of the intrinsic frequency dominant in an alternating current rotating machine. There is the possibility that the magnetic sound would rather be increased by the superimposition of current or that almost no effect of reduction of the magnetic sound could be obtained.
Namely, a person skilled in the art could easily have conceived of changing the current in some way so as to change the magnetic sound relating to the electro-magnetic force created by that current, but would not have considered the current waveforms to be given for reducing the magnetic sound, particularly the frequencies thereof, so it would have been difficult to actually reduce the magnetic sound with a good precision. This problem becomes still further difficult in the reduction of for example the magnetic sound of an alternating current rotating machine for generating a driving torque in which the driving state changes without interruption.